Japanese Hops
Humulus japonicus
Hemp family (Cannabinaceae)

Description: This annual vine is 3-20' long, branching occasionally. The twining habit of its stems allows this vine to climb adjacent vegetation and fences; otherwise, it sprawls across the ground. The rather stout stems are light green to reddish purple and longitudinally ridged. Along the ridges of each stem, there are rows of stiff prickly hairs. Pairs of opposite leaves occur at intervals along each stem. These leaves are up to 6" long and 6" across and palmately divided into 5-7 lobes. Each lobe is oblanceolate or elliptic in shape with coarsely serrated margins. The upper surface of each leaf is medium green and sparsely covered with short rough hairs, while the lower surface has stiff prickly hairs along the major veins. The rather stout petioles are as long as, or a little shorter than, their leaves; these petioles are light green and covered with stiff prickly hairs. Japanese Hops is dioecious, with female (pistillate) and male (staminate) flowers produced on separate vines. The female inflorescence is a short spike of flowers and their bracts. This spike becomes globoid with age and tends to nod downward, spanning about 1" across. The appearance of each spike is dominated by several overlapping pistillate bracts; at the base of each bract, there is a pair of inconspicuous female flowers. Initially, the pistillate bracts are narrowly deltoid in shape, but they later enlarge in size and become deltoid with recurved tips. These bracts are light green, hairy, and strongly ciliate along their smooth margins. Each female flower has a divided style and an inconspicuous calyx that surrounds the developing ovary; there are no petals. The male inflorescence consists of a panicle of flowers up to 10" long and half as much across; this inflorescence is more or less erect with ascending to widely spreading lateral branches. The hairy branches of each panicle are light green to pale red (usually the latter); there is a pair of small linear bracts at the base of each branch. Individual male flowers are about 1/8" (3 mm.) across, consisting of 5 spreading sepals, 5 anthers, and no petals; they usually droop downward from the branches. The sepals are elliptic in shape and vary in color from light green to pale red (usually the latter). Both male and female inflorescences are axillary (non-terminal). The blooming period occurs from mid-summer to early fall and lasts about 2 months. The flowers are wind-pollinated. Each female flower produces a single seed that is ovoid and flattened. This vine reproduces by reseeding itself. It often forms dense colonies of overlapping vines.

Cultivation: Japanese Hops prefers full to partial sun and moist to dry-mesic conditions. It tolerates almost any kind of soil (sandy, gravelly, loamy, or full of clay), but grows most vigorously in moist fertile loam. This vine can spread aggressively by reseeding itself. While this plant is an annual in Illinois, it can survive the winter in warmer climates.

Range & Habitat: Japanese Hops is occasional in some areas of Illinois (particularly the central and northern regions of the state), but it is still absent from other areas (see Distribution Map). However, this species is still spreading into new areas and it may eventually become a major weed. As the common name suggests, this species is native to East Asia; it was probably introduced into the United States as an ornamental plant. Habitats include disturbed weedy meadows, banks of rivers and drainage ditches, banks along ponds, open areas along railroads and roadsides, construction sites, and waste ground. Recently disturbed habitats with exposed open ground are especially preferred; this species also seems to prefer open slopes along bodies of water where there is little or no mowing.

Faunal Associations: While photographing this vine, I observed several honeybees collecting pollen from the staminate flowers; other bees and flower flies are attracted to the pollen as well, although none of these insects are effective at cross-pollination (they don't visit the female flowers). Most hops-related insect records are for the more common Humulus lupulus (American Hops), although these same insects may feed on the closely related Japanese Hops; the Insect Table lists some of these insects.

Photographic Location: Judge Webber Park in Urbana, Illinois, where some recent construction activity occurred. This species also grows along the banks of ponds and drainage ditches in the area.

Comments: Even though they are superficially similar in appearance, it is relatively easy to distinguish Japanese Hops from Humulus lupulus (American Hops). Japanese Hops has leaves with mostly 5-7 lobes, while American Hops has leaves with 3 lobes or none. Because of the color of its sepals and branches, the staminate inflorescence of Japanese Hops is often pale red in appearance, while the staminate inflorescence of American Hops is usually pale yellow. The pistillate spikes of Japanese Hops are globoid in shape and they have recurved deltoid bracts (triangular-shaped with slender tips that bend backward) that are hairy. In contrast, the pistillate spikes of American Hops are ovoid in shape and they have hairless ovate bracts with blunt tips that are straight or bend slightly inward.